47.13 docker init: generate a Dockerfile, Compose file and .dockerignore

Example 1: Check the version

Type this:

docker init --version

You will see:

Version:    v1.4.0
Git commit: 6d1d8b4

What it means: The plugin is installed. Your version number may differ.

Example 2: When it is missing

Type this:

docker init

You will see:

docker: 'init' is not a docker command.
See 'docker --help'

What it means: Install Docker Desktop, or update Docker Engine from the official repo as in lesson 47.5.

Example 3: See the help

Type this:

docker init --help

You will see:

Usage:  docker init [OPTIONS]
Creates Docker-related starter files for your project

What it means: Short help. The real work is in the questions it asks.

Example 4: Run it in an empty folder

Type this:

mkdir empty && cd empty
docker init

You will see:

? What application platform does your project use?
  Go
  Python
  Node
  Rust
  ASP.NET Core
  PHP with Apache
  Java
> Other

What it means: In an empty folder it cannot detect anything, so it asks you.

Example 5: Let it detect

Type this:

cd ../shop-api && ls
docker init

You will see:

package.json  server.js
? What application platform does your project use? Node (detected)

What it means: It saw package.json and guessed Node.

Example 6: Leave without writing

Type this:

(press Ctrl+C at any question)

You will see:

^C
(no files written)

What it means: Nothing is written until you answer every question.

Example 7: Make a tiny app

Type this:

mkdir shop-api && cd shop-api
npm init -y > /dev/null
npm install express@4 > /dev/null
printf 'const e=require("express")();\ne.get("/",(q,r)=>r.send("Paytm shop API"));\ne.listen(3000);\n' > server.js
ls

You will see:

node_modules  package-lock.json  package.json  server.js

What it means: A real project with a lock file.

Example 8: Run docker init

Type this:

docker init

You will see:

? What application platform does your project use? Node
? What version of Node do you want to use? 20
? Which package manager do you want to use? npm
? What command do you want to use to start the app? node server.js
? What port does your server listen on? 3000
CREATED: .dockerignore
CREATED: Dockerfile
CREATED: compose.yaml
CREATED: README.Docker.md

What it means: Four files are created.

Example 9: See the files

Type this:

ls -a

You will see:

..  .dockerignore  Dockerfile  README.Docker.md  compose.yaml  node_modules  package-lock.json  package.json  server.js

What it means: README.Docker.md has short notes on how to build and deploy.

Example 10: Look at the important lines

Type this:

grep -E '^(FROM|ENV|RUN|USER|COPY|EXPOSE|CMD)' Dockerfile

You will see:

FROM node:20-alpine
ENV NODE_ENV production
RUN --mount=type=bind,source=package.json,target=package.json \
USER node
COPY . .
EXPOSE 3000
CMD node server.js

What it means: alpine keeps it small. USER node means it does not run as root. EXPOSE notes the port.

Example 11: The cache mount line

Type this:

grep -A3 'RUN --mount' Dockerfile

You will see:

RUN --mount=type=bind,source=package.json,target=package.json \
    --mount=type=bind,source=package-lock.json,target=package-lock.json \
    --mount=type=cache,target=/root/.npm \
    npm ci --omit=dev

What it means: A cache mount keeps the npm download cache between builds, so the second build is faster.

Example 12: Comments explain each part

Type this:

grep -c '^#' Dockerfile

You will see:

22

What it means: Many comment lines. Read them; they teach good habits.

Example 13: The ignore list

Type this:

grep -v '^#' .dockerignore | grep -v '^$' | head -6

You will see:

**/.git
**/.env
**/node_modules
**/Dockerfile*
**/compose*
README.md

What it means: node_modules and .env stay out. Your local secrets never enter the image.

Example 14: The compose file

Type this:

grep -v '^ *#' compose.yaml | grep -v '^$'

You will see:

services:
  server:
    build:
      context: .
    environment:
      NODE_ENV: production
    ports:
      - 3000:3000

What it means: One service named server, built from this folder, on port 3000.

Example 15: Commented database example

Type this:

grep -c 'db:' compose.yaml

You will see:

2

What it means: A Postgres example is included as comments. Remove the # marks when you need it.

Example 16: Start it

Type this:

docker compose up --build -d

You will see:

[+] Building 14.2s (12/12) FINISHED
[+] Running 2/2
 ✔ Network shop-api_default     Created
 ✔ Container shop-api-server-1  Started

What it means: Built and running in the background.

Example 17: Test it

Type this:

curl -s localhost:3000

You will see:

Paytm shop API

What it means: The API answers from the container.

Example 18: Check the user and size

Type this:

docker compose exec server whoami
docker images shop-api-server --format '{{.Size}}'

You will see:

node
141MB

What it means: It runs as node, not root, and the image is small.

Example 19: Make the app

Type this:

mkdir menu && cd menu
printf 'flask==3.0.3\ngunicorn==22.0.0\n' > requirements.txt
printf 'from flask import Flask\napp=Flask(__name__)\n@app.get("/")\ndef h(): return "Swiggy menu"\n' > app.py

You will see:

(no output)

What it means: Two files: dependencies and code.

Example 20: Run docker init

Type this:

docker init

You will see:

? What application platform does your project use? Python
? What version of Python do you want to use? 3.12
? What port do you want your app to listen on? 8000
? What is the command you use to run your app? gunicorn 'app:app' --bind=0.0.0.0:8000
CREATED: Dockerfile ...

What it means: gunicorn is a production web server for Python.

Example 21: Build and test

Type this:

docker compose up --build -d
curl -s localhost:8000

You will see:

Swiggy menu

What it means: Same flow, different language.

Example 22: Make the app

Type this:

mkdir quote && cd quote
go mod init quote
printf 'package main\nimport ("fmt";"net/http")\nfunc main(){http.HandleFunc("/",func(w http.ResponseWriter,r *http.Request){fmt.Fprint(w,"Zerodha quote")});http.ListenAndServe(":8080",nil)}\n' > main.go

You will see:

go: creating new go.mod: module quote

What it means: A one-file web server.

Example 23: See the two stages

Type this:

docker init   # answer Go, 1.23, ., 8080
grep -E '^FROM' Dockerfile

You will see:

FROM --platform=$BUILDPLATFORM golang:1.23 AS build
FROM alpine:latest AS final

What it means: Stage one has the Go compiler; stage two does not.

Example 24: Compare sizes

Type this:

docker compose up --build -d
docker images quote-server --format '{{.Size}}'
curl -s localhost:8080

You will see:

19.8MB
Zerodha quote

What it means: Under 20 MB, because the compiler is not in the final image.

Example 25: Run it again

Type this:

docker init

You will see:

WARNING: The following Docker files already exist in this directory:
  - Dockerfile
  - compose.yaml
? Do you want to overwrite them? No

What it means: Answer No to keep your files.

Example 26: Back up first

Type this:

cp Dockerfile Dockerfile.mine
docker init   # answer Yes to overwrite
diff Dockerfile.mine Dockerfile | head -3

You will see:

5c5
< FROM node:18
> FROM node:20-alpine

What it means: Now you can compare and keep the best parts.

Example 27: Keep files in Git

Type this:

git add Dockerfile compose.yaml .dockerignore
git commit -m 'Add Docker files'

You will see:

[main 3e4f5a6] Add Docker files
 3 files changed, 98 insertions(+)

What it means: Commit them so the whole team uses the same files.

Example 28: Pin the base image

Type this:

sed -i 's/FROM alpine:latest AS final/FROM alpine:3.20 AS final/' Dockerfile
grep 'AS final' Dockerfile

You will see:

FROM alpine:3.20 AS final

What it means: latest can change any day. A fixed tag keeps builds repeatable.

Example 29: Add a health check

Type this:

printf 'HEALTHCHECK --interval=30s CMD wget -qO- localhost:8080 || exit 1\n' >> Dockerfile
docker compose up --build -d
sleep 35; docker ps --format '{{.Names}} {{.Status}}'

You will see:

quote-server-1 Up 40 seconds (healthy)

What it means: Docker now checks the app every 30 seconds.

Example 30: Enable the database block

Type this:

grep -n '# *db:' compose.yaml

You will see:

22:#   db:

What it means: Remove the # marks from that block to add Postgres, then add a password through a secret file, never in the file itself.

Example 31: Stop each project

Type this:

cd ~/shop-api && docker compose down

You will see:

[+] Running 2/2
 ✔ Container shop-api-server-1  Removed
 ✔ Network shop-api_default     Removed

What it means: Run it in each project folder.

Example 32: Remove images too

Type this:

docker compose down --rmi local

You will see:

✔ Image quote-server Removed

What it means: --rmi local removes images that Compose built.

Example 33: Check

Type this:

docker ps --format '{{.Names}}'

You will see:

(no output)

What it means: Nothing is left running.